1. Field of the Invention
The present invention relates to the field of optical communication systems, and more particularly, to an improved fiber optic cable suited for use in a long distance optical communication system.
2. Art Background
Since the original invention of the laser, optical communication by laser beam has been one of the principal applications envisioned for this device. Lasers provide light beams having extreme directionality compared to beams produced by other light sources or microwave antennas. In addition, the amount of information which may be transmitted over an electromagnetic wave is proportional to the bandwidth of the wave. Since some lasers operate with very large modulation bandwidths, it is theoretically possible to transmit thousands of color television broadcasts over a single laser beam. Although bandwidth and directionality are two inherent advantages of laser communications systems, a number of disadvantages apply to laser communications. The usefulness of a laser communications system is generally limited by factors such as the necessity of electro-optic detectors, and the ease with which information can be imposed upon the beam itself. Moreover, atmospheric conditions may significantly attenuate the optical radiation of the laser, and laser light does not reflect well or penetrate through objects.
An alternative to atmospheric propagation is the use of optical fibers comprised of glass or plastic to confine and direct the laser beam. The use of optical fibers permits efficient communication within dense population areas and eliminates the complex maze of wires and coaxial cables which have been required for electronic communication.
For long distance communication, a repeater for each channel within a fiber optic bundle is required to correct distortion of the optical signal caused by differential propagation delays, attenuation and noise. However, the use of repeaters interspersed along fiber optic cable segments in a communication system requires that the repeaters be provided with power for their circuits, significantly increases the cost of installation and decreases system reliability. Attenuation, or power loss, in an optical fiber is caused by a variety of processes, and various techniques have been attempted to reduce the attenuation, and therefore the need for repeaters, in a fiber optic communication system. (See: Drexhage, "Infrared Optical Fibers", Scientific American, Nov. 1988, pg. 110; and, O'Shea, "An Introduction to Lasers and Their Applications", Chapter 8, Addison-Wesley, Dec. 1978.)
As will be described, the present invention overcomes the disadvantages associated with prior art fiber optic cables, and discloses a fiber optic cable which eliminates the need for repeaters by effectively lasing the entire cable length. The cable is "pumped" to achieve lasing along its length using one of a variety of pumping mechanisms, as disclosed herein.